Coal is an abundant energy source and is expected to remain the dominant fuel for power generation at least through the first half of this century. Today coal-fired power plants must meet stringent environmental requirements with regard to the byproducts they produce.
One of the byproducts of power plants and turbine engines is exhaust gas, commonly known as flue gas. This gas may contain components which are harmful to the environment, such as oxides of nitrogen (NOx). The production of NOx can occur when fossil fuels are combusted such as in refinery heaters, steam boilers, etc.
Plants which burn coal and/or other fuels in many cases are required to control SO2, NOx, and CO and particulate matter. In addition on limits to these byproducts, modern plants may be required to control emission of mercury, SO3/H2SO4, condensable particulate matter, various trace metals and/or acid gases.
Presently for SO3/H2SO4, condensable PM control, power plants often use wet electrostatic precipitator (WESP), or alkali sorbent injection without gas coolers, or a combination of the two to meet emission limits, with high capital and operating costs.
A wet ESP operates at temperatures below the sulfuric acid dew point and collects the H2SO4 and other condensable matters in the form of liquid mists. Therefore, all internal components of WESP that are exposed to the gas stream, as well as the flue ducts connecting the WESP must be made of corrosion-resistant material, generally very expensive alloys. The WESP has become one of the most expensive devices for power plant pollution control. Additionally, WESP has higher power consumption and significantly reduces plant net output efficiency. Alternately various alkali sorbent injection systems installed at lower capital costs than WESPs. However, they require continuous injection of sorbent material (typically sodium, magnesium or calcium based) and therefore add significantly to plant operating cost. Sorbent injection also introduces external substance to combustion byproducts (flyash) and therefore can affect its utilization options.
In some known systems gas coolers have been used to cool the flue gas prior to processing by a dry electrostatic precipitator (ESP) or fabric filter (FF) used to remove particulate matter. Energy may be recovered as part of the cooling process and used elsewhere in the system, e.g., to re-heat the flue gas after processing and prior to emission through a smoke stack thereby reducing the amount of visible condensate at the top of the stack and giving the power plant a much cleaner looking and less visible output.
The cooling of the gas prior to ESP or FF processing facilitates pollution control and allows for the use of dry as opposed to wet flue gas processing techniques. However the cooling has the side effect of producing condensate which tends to be rather corrosive.
One known system which uses a selective catalytic reduction (SCR) process to treat flue gas is shown in FIG. 1. The known system 100 includes a boiler 102, an SCR module 106, an air heater 110, a gas cooler 114, a particulate removal device in the form of an electrostatic precipitator and/or fabric filter (ESP/FF) 120. The system 100 also includes an induced draft fan (IDF) 124 for forcing the flue gas into a flue gas inlet 128 of a wet flue gas desulfurization device 130 which is a vessel where the flue gas is treated, e.g., using limestone or other reactants prior to the gas being discharged through a smoke stack 140. As part of the treatment process, condensate and/or limestone is pumped by pump 136 though pipe 134 and nozzles 138 which are used to add reactants and/or condensate to the flue gas as it passes through the WFGD 130.
Depending on the amount of condensation caused by the use of the gas cooler 114 and various conditions including the weather which may impact humidity, the known system 100 may suffer from clogging of the gas cooler due to clumping of wet ash. This can occur when the amount of dry material in the flue gas stream is insufficient to absorb all of the condensate. In addition, as discussed above, the known system 100 has the disadvantage of being costly to implement due to the use of stainless steel and/or other corrosion resistant materials to implement portions of the gas cooler 114 such as the cooling tube bundles shown by the wavy line between the fluid input 142 and fluid output 118 through which a cooling fluid is passed.
FIG. 2 illustrates a known system 200 similar to that of FIG. 1. Elements of FIGS. 1 and 2 which are the same are indicated using the same reference numbers. In FIG. 2, energy extracted by the gas cooler 114 is recovered by an energy recovery module 210 which operates in combination with pump 204. The pump 204 pumps cooling fluid from its inlet 208, through its outlet 44. The cooling fluid circulates through gas cooler 114 absorbing heat from the flue gas before passing though outlet pipe 206 to the input of the energy recovery module 210. The energy recovery module 210 is implemented as a heat exchange unit with heat being transferred from the cooling fluid to a heat transfer solution which exits the energy recovery module 210 at outlet 212. In the known system 200 the recovered energy is used to heat the flue gas after treatment in the WFGD thereby raising the outlet temperature of the flue gas exhausted through smoke stack 140. The heating of the flue gas prior to exhausting has the advantage of reducing the visible condensation at the smoke stack outlet. However, it does not improve overall efficiency since the energy used to heat the flue gas achieves what is largely a cosmetic result while requiring heat energy. While not necessary, it would be desirable if a more beneficial use, e.g., in terms of energy efficiency, could be developed for the energy recovered from the gas cooler.
Unfortunately, as noted above, existing systems which use gas coolers tend to be expensive due to the cost of corrosion resistant materials, such as stainless steel, used to implement the gas coolers. In addition, with the conventional systems which use gas coolers, there is a potential for clogs and/or other problems due to combustion by products clumping and blocking the flue gas path as the combustion byproducts become wet due to condensation which occurs during the cooling process. As mentioned above, this is because wet materials may build up on the heat exchange surfaces of the gas cooler blocking the flow of the flue gas. The cleaning of such suffices when a clog occurs can interfere with normal system operation and/or result in costly service.
Given that systems using gas coolers can be simpler to implement than systems using WESP, it would be desirable if methods and/or apparatus could be developed which would address one or more of the known problems including, for example, the requirement that the flue gas cooler be implemented using large amounts of corrosion resistant materials such as stainless steel and the tendency for combustion byproducts to become wet, clump, and block the flow of flue gas. Thus, it would be beneficial if method and/or apparatus were developed which reduced the amount of corrosion resistant materials needed to implement a reliable flue gas cooler and/or which reduced the tendency for clogs and/or clumping to occur. While not critical, it would also be beneficial if new uses for the recovered energy which can be obtained from the use of a flue gas cooler were developed which would allow for improved overall energy efficiency as compared to the known systems discussed above.